According to a type of accelerometer using a piezoresistive effect, may include a box shape of seismic mass (i.e. movable mass), contained in a cavity of a silicon base member. The movable mass is suspended by beams on which a piezoresistance are formed, so that a stress is applied to the piezoresistance in response to movement of the movable mass. The variation of stress applied to the piezoresistance is detected as a variation of resistance.
The above-described mass is required to move freely, however, if the mass over-moves, the sensor might be broken or damaged. For inhibiting over-move of the mass, a stopper is arranged above the moving mass with a predetermined space (distance).
In fabrication of such an acceleration sensor, an insulating layer is formed on a silicon substrate and an active layer is formed on the insulating layer. A plurality of piezoresistances is formed on the active layer. A stopper is formed as a part of the active layer. According to a conventional acceleration sensor described in Japanese Patent Application Publication No. 2004-198243, stoppers are provided with holes and the active layer is provided with a groove separating a movable mass, beams, a stationary frame and the stoppers from each other in a horizontal plane.
The holes are formed on the stopper regions in the same process as forming the groove of the active layer. After that, the insulating layer, located between the stoppers and the movable mass, is selectively removed to separate the stopper from the movable mass in a vertical direction by a wet-etching process. Such a wet-etching process is carried out through the holes on the stoppers.
According to the above described conventional acceleration sensor, however, electronic elements including the piezoresistances and conductive wiring formed on the active layer may be damaged in the wet-etching process, because a wet-etching process takes a long time to be completed.